A standard television video signal is made up of relatively short duration, fixed amplitude, negative-going, line (horizontal) synchronizing pulses, between which a varying amplitude, image brightness-representative (luminance) video signal occurs in association with a particular image scanning line. The horizontal sync pulses occupy a blanking interval which includes, in addition to the sync pulse, a "back porch" or relatively fixed amplitude portion, the amplitude of which is defined as the reference black level of the image. The sync pulses extend into the blacker-than-black amplitude region of the video signal.
In the case of color television signals, a color synchronizing burst or short duration, continuous wave sample of a color subcarrier (e.g. 3.58 MHZ) is superimposed on the back porch of the composite video waveform. Color difference signals, which are produced as modulation components of the suppressed subcarrier waveform, also are spaced between the periodic burst signals during the line scanning intervals.
In the processing of the television composite video signals, numerous circumstances arise in which it is necessary to fix the reference black level of the demodulated luminance signal at a particular d-c voltage level that is appropriate for a particular part of the signal processing system. Furthermore, it is frequently necessary to d-c restore the demodulated color difference signals to a proper d-c level before they are recombined with luminance information to produce desired red (R), green (G) and blue (B) drive signals for application to an associated display device.
A particular problem arises in cases where significant segments of signal processing are to be accomplished making use of existing integrated circuit chips. In that case, since signals are modified within the confines of a chip and only a limited number of access points are provided to bring certain signals out of and into any single chip, manipulation of signals in a unique manner outside of the chips may be required. Furthermore, where signals are processed outside a chip or are coupled from chip to chip, a-c coupling is usually required, thereafter necessitating d-c restoring before ultimate application to an image reproducing device.
For example, certain single chip TV processors (e.g. Toshiba Type TA8680) do not include provisions to blank or remove the color burst signal from the modulated color difference signals prior to detection. As a result, when the color difference signals are demodulated within the processor chip, a d-c "burst offset" is produced above the reference level of the demodulated color difference signals during the back porch interval. The presence of this burst offset vitiates against d-c restoration of the color difference signals during the preferred back porch interval since that level is contaminated by the presence of the burst offset. In that case, d-c restoration of the color difference signals during the immediately adjacent sync pulse interval would be called for. However, the accompanying luminance signals include blacker-than-black sync tips during the sync interval, and the presence of these sync tips would produce an imbalance between the luminance and chrominance d-c levels if a sync interval clamp were employed in connection with both the luminance and the demodulated color difference signals. In order to permit d-c restoration of both the demodulated luminance and chrominance signals during the same time interval (i.e. by means of a single gating pulse applied to a single chip terminal), additional special processing of the luminance signal or the color difference signal is required.
Circuits and systems are known for processing combined luminance and sync signals so as to separate the two and thereby produce luminance signals from which sync has been stripped (see, for example, U.S. Pat. No. 4,628,361, "Video Synchronizing Signal Separator" granted Dec. 9, 1986 to Sam Andreas). Such circuits include, in general, a unidirectionally conducting device which is biased to reject the sync tips and to pass luminance information.
Additional circuits are known for clamping the reference black level occurring during the back porch interval to a desired d-c level (see, for example, U.S. Pat. No. 4,424,528, "Video Circuit", granted Jan. 3, 1984 to J. A. Karlock et al.) and thereafter separating the sync pulses from the clamped luminance.
It is also known to separate sync pulses from video by clamping the sync tips to a reference voltage such as ground and thereafter clipping all amplitude components above a predetermined level to leave only the sync pulses for further processing (see, for example, U.S. Pat. No. 4,081,833, "Synchronizing Signal Separating Circuit For Video Signal Processing", granted Mar. 28, 1978 to H. Akiyama; U.S. Pat. No. 4,296,437, "Clamping Circuit for a Video Signal", granted Oct. 20, 1981 to M. F. A. M. Geurts; U.S. Pat. No. 4,489,349, "Video Brightness Control Circuit", granted Dec. 18, 1984 to T. Okada).
Despite the foregoing developments, there remains a need for a simple, accurate and reliable combined video clamp and sync separator circuit for providing an accurately referenced video signal free of sync which may thereafter be a-c coupled to a subsequent processing circuit including a sync interval d-c restorer.
Furthermore, a single circuit for providing the foregoing video signal free of sync and a separate output of sync free of video without the need for adjustment of circuit parameters or voltage levels is particularly desirable.